Exhaust emission control device of internal combustion engine

ABSTRACT

At the start of operation of an internal-combustion engine(S 10 ,S 12 ,S 14 ), an exhaust flow is restrained (to raise the exhaust pressure) (S 18 ), secondary air is supplied (S 20 ), and the combustion air-fuel ratio (A/F) is set within the range of rich air-fuel ratios (S 16 , S 22 ).

TECHNICAL FIELD

[0001] The present invention relates to an exhaust emission controldevice of an internal-combustion engine, and more specifically, to atechnique for reducing the delivery of harmful substances from aninternal-combustion engine by using secondary air supply means.

BACKGROUND ART

[0002] An exhaust emission control technique is known that utilizesreaction on a catalyst in order to reduce harmful substances (includingsmoke, NOx, etc. as well as unburned substances such as HC, CO, H₂,etc.) in exhaust gas. Also known is a secondary air technique in which acatalyst is activated early by supplying secondary air to an exhaustport.

[0003] In some cases, however, harmful substances that are dischargedbefore the activation of the catalyst amount to 90% of the totaldelivery in a cold mode, and arouse a critical problem.

[0004] Described in Jpn. Pat. Appln. KOKAI Publications Nos. 3-117611and 4-183921, therefore, is a developed technique such that the exhaustpressure is raised in the cold state to activate the catalyst early. Asis described in Jpn. Pat. Appln. KOKAI Publication No. 8-158897, forexample, a technique is developed such that the catalyst is activatedearly by raising the exhaust pressure and supplying secondary air in thecold state.

[0005] According to an experiment conducted by the applicant hereof, itwas confirmed that reaction in an exhaust system can be accelerated toenhance the exhaust gas purifying capability and improve the exhaustemission control efficiency by using the secondary air technique incombination with any of the exhaust pressure raising techniquesdescribed above.

[0006] However, an investigation made afterward by the applicant thereofindicates that the exhaust emission control performance lowers if thesecondary air is supplied and the exhaust pressure is raised in the coldstate with the combustion air-fuel ratio of the internal-combustionengine kept at a normal combustion air-fuel ratio for the case where theexhaust pressure is not raised.

[0007] If the fuel injection is carried out in an intake stroke so thatthe combustion air-fuel ratio in the cold state is a rich air-fuelratio, in the case of a cylinder-injection internal-combustion engine,moreover, exhaust gas that contains plenty of hydrocarbon (HC), anunburned fuel, is discharged into exhaust passages. Since HC reacts lesseasily than carbon monoxide (CO) does, in general, the exhaust emissioncontrol performance is not improved very much although the secondary airsupply and exhaust pressure raising are carried out.

[0008] In order to accelerate reaction with the secondary air in theexhaust system satisfactorily, on the other hand, the exhaust pressureshould be raised to about 700 mmHg (933 hPa). Since the dischargepressure of an air pump that is generally used for the purpose ofsecondary air supply is about 150 mmHg (200 hPa), however, the air pumpis expected to be considerably improved in performance. If the air pumpis large-sized, there is the problem of increase in cost as well as indriving power consumption.

[0009] In this case, an attempt may be made to increase the internal EGRby raising the exhaust pressure and changing the overlap opening of anexhaust valve without using any secondary air, as is described in Jpn.Pat. Appln. KOKAI Publications Nos. 5-231195 and 8-158897, for example.However, the exhaust pressure raising technique and the increase of theinternal EGR alone cannot satisfactorily accelerate the reaction in theexhaust system. Thus, it is hard to improve the exhaust emission controlefficiency and fully activate the catalyst early.

DISCLOSURE OF THE INVENTION

[0010] The object of the present invention is to provide an exhaustemission control device of an internal-combustion engine, in whichsecondary air can be securely fed into an exhaust system with use of alow-priced configuration despite a rise in exhaust pressure, andreaction in the exhaust system can be satisfactorily accelerated at thestart of operation of the internal-combustion engine, so that theexhaust emission control efficiency can be improved.

[0011] In order to achieve the above object, an exhaust emission controldevice of an internal-combustion engine according to the presentinvention comprises exhaust flow control means for restraining anexhaust flow so as to enhance the effect of reduction of the delivery ofharmful substances at the start of operation of the internal-combustionengine, secondary air supply means for supplying secondary air to anexhaust system of the internal-combustion engine at the start ofoperation of the internal-combustion engine, and air-fuel ratio controlmeans for adjusting the combustion air-fuel ratio of theinternal-combustion engine to a rich air-fuel ratio at the start ofoperation of the internal-combustion engine.

[0012] In the case where the secondary air is supplied and the exhaustflow is restrained (to raise the exhaust pressure) at the start ofoperation of the internal-combustion engine, therefore, reaction in theexhaust system (including a combustion chamber, exhaust port, exhaustmanifold, exhaust pipes, etc.) can be accelerated to improve the exhaustemission control efficiency by setting the combustion air-fuel ratio(air-fuel ratio before the secondary air supply) within the range ofrich air-fuel ratios.

[0013] In the exhaust emission control device of the internal-combustionengine of the present invention, moreover, the air-fuel ratio controlmeans controls the combustion air-fuel ratio of the internal-combustionengine so that the combustion air-fuel ratio is not lower than acombustion limit air-fuel ratio and not higher than 13.

[0014] In the case where the secondary air is supplied and the exhaustflow is restrained (to raise the exhaust pressure) at the start ofoperation of the internal-combustion engine, therefore, the exhaustemission control performance can be optimized by setting the combustionair-fuel ratio (air-fuel ratio before the secondary air supply) withinthe range of relatively high rich air-fuel ratios not lower than thecombustion limit air-fuel ratio and not higher than 13.

[0015] Referring to FIG. 5, there are shown relations between thecombustion air-fuel ratio (combustion A/F) before the secondary airsupply and the HC delivery for each exhaust pressure as the results ofmeasurement in 10 seconds after the start of operation of theinternal-combustion engine. In this drawing, a two-dot chain line,dashed line, broken line, and full line represent cases of exhaustpressures of 0 mmHg (0 hPa), 300 mmHg (400 hPa), 500 mmHg (667 hPa), and700 mmHg (933 hPa), respectively. If the exhaust pressure rises in thismanner, the HC delivery is reduced in general within the range of richair-fuel ratios not higher than 13, in particular.

[0016] This phenomenon is supposed to occur for the following reason.The rise of the exhaust pressure heightens the exhaust gas density. Asthe air-fuel ratio is enriched, the quantity of unburned substancesdischarged from the combustion chamber increases correspondingly, theprobability of reaction in the exhaust system (including the combustionchamber, exhaust port, exhaust manifold, and exhaust pipes) increases,and the exhaust gas flows back from the exhaust port into the combustionchamber. As the gas in the combustion chamber is stirred, oxidation ofunburned HC and the like are accelerated.

[0017] In the exhaust emission control device of the internal-combustionengine of the present invention, moreover, the secondary air supplymeans supplies the secondary air so that the exhaust air-fuel ratioobtained after the secondary air supply is a lean air-fuel ratio.

[0018] Thus, the reaction in the exhaust system can be furtheraccelerated to improve the exhaust emission control efficiencyadditionally by supplying the secondary air so that the combustionair-fuel ratio is a rich air-fuel ratio and the exhaust air-fuel ratioafter the secondary air supply is a lean air-fuel ratio.

[0019] In the exhaust emission control device of the internal-combustionengine of the present invention, furthermore, the secondary air supplymeans supplies the secondary air so that the exhaust air-fuel ratioobtained after the secondary air supply ranges from 18 to 22.

[0020] Thus, the exhaust emission control performance can be furtheroptimized by supplying the secondary air so that the combustion air-fuelratio is a rich air-fuel ratio and the exhaust air-fuel ratio after thesecondary air supply ranges from 18 to 22.

[0021] Referring to FIG. 6, there is shown the relation between theexhaust air-fuel ratio (exhaust A/F) after the secondary air supply tothe exhaust system and the HC delivery under the exhaust pressure of 700mmHg (933 hPa) as the result of an experiment in 50 seconds after thestart of operation of the internal-combustion engine. If the exhaustpressure is raised in this manner, the HC delivery has its minimum whenthe exhaust air-fuel ratio is 20 or thereabout, and can be speciallylowered when the exhaust air-fuel ratio ranges from 18 to 22.

[0022] In the exhaust emission control device of the internal-combustionengine of the present invention, furthermore, the internal-combustionengine is a multi-cylinder internal-combustion engine, and the secondaryair supply means supplies the secondary air by stopping fuel supply tosome of cylinders or carrying out lean-A/F operation.

[0023] If the secondary air is supplied by stopping the fuel supply tosome of the cylinders or carrying out the lean-A/F operation, therefore,plenty of oxygen is discharged from some cylinders, while greatquantities of unburned substances are discharged with a rich air-fuelratio from the other cylinders. Thus, when the exhaust pressure israised, the unburned substances and oxygen react satisfactorily in theexhaust system, so that the exhaust emission control efficiency can beimproved with ease.

[0024] In the exhaust emission control device of the internal-combustionengine of the present invention, moreover, the internal-combustionengine is a multi-cylinder internal-combustion engine and comprises twoexhaust passages provided independently for each of two cylinder groupsinto which cylinders of the multi-cylinder internal-combustion engineare divided and communicating channels connecting the two exhaustpassages, and the secondary air supply means includes exhaust oxygenquantity increasing means for increasing the quantity of oxygen inexhaust gas discharged from one of the two cylinder groups, and suppliesthe secondary air as the exhaust control means restrains the exhaustflow so that the degree of restraint of the flow of the exhaust gasdischarged from the one cylinder group is higher than the degree ofrestraint of the flow of the exhaust gas discharged from the othercylinder group when the quantity of oxygen in the exhaust gas dischargedfrom the one cylinder group is increased by means of the exhaust oxygenquantity increasing means.

[0025] Thus, the exhaust oxygen quantity increasing means is used toincrease the quantity of oxygen in the exhaust gas discharged from theone cylinder group (e.g., to perform lean-A/F operation), and theexhaust flow is restrained so that the degree of restraint of the flowof the exhaust gas discharged from the one cylinder group (degree ofrise of the exhaust pressure) is higher than the degree of restraint ofthe flow of the exhaust gas discharged from the other cylinder group. Asthis is done, the exhaust pressure of the oxygen-rich exhaust gasdischarged from the one cylinder group becomes higher than the exhaustpressure of the exhaust gas discharged from the other cylinder group.Based on the resulting pressure difference, the oxygen-rich exhaust gasis supplied as the secondary air from the exhaust passages of the onecylinder group to the exhaust passages of the other cylinder groupthrough the communicating channels.

[0026] Thus, in the case where the exhaust flow is restrained to raisethe exhaust pressure, the secondary air can be supplied securely toaccelerate the reaction in the exhaust system without using anylarge-capacity secondary air pump, so that the exhaust emission controlefficiency can be improved with use of a simple configuration withoutany increase in cost.

[0027] In the exhaust emission control device of the internal-combustionengine of the present invention, furthermore, the internal-combustionengine is a multi-cylinder internal-combustion engine and comprises twoexhaust passages provided independently for each of two cylinder groupsinto which cylinders of the multi-cylinder internal-combustion engineare divided and communicating channels connecting the two exhaustpassages, and the secondary air supply means includes exhaust oxygenquantity increasing means for increasing the quantity of oxygen inexhaust gas discharged from one of the two cylinder groups and an airpump attached to the communicating channels and capable of force-feedingthe exhaust gas from the exhaust passages of the one cylinder group tothe exhaust passages of the other cylinder group.

[0028] Thus, the exhaust flow is restrained (to raise the exhaustpressure) by means of the exhaust flow control means, the quantity ofoxygen in exhaust gas discharged from the one cylinder group isincreased by means of the exhaust oxygen quantity increasing means, andthe air pump is actuated. As this is done, the oxygen-rich exhaust gasis supplied as the secondary air from the exhaust passages of the onecylinder group to the exhaust passages of the other cylinder groupthrough the communicating channels despite the rise in the exhaustpressure attributable to the restraint of the exhaust flow.

[0029] Thus, in the case where the exhaust flow is restrained to raisethe exhaust pressure, the secondary air can be supplied securely toaccelerate the reaction in the exhaust system by means of asmall-capacity air pump, not a large-capacity secondary air pump, sothat the exhaust emission control efficiency can be improved with use ofa simple configuration without any increase in cost.

[0030] In the exhaust emission control device of the internal-combustionengine of the present invention, furthermore, the exhaust oxygenquantity increasing means stops fuel supply to one of the two cylindergroups or performs lean-A/F operation.

[0031] Thus, the quantity of oxygen discharged from the on cylindergroup can be easily increased to feed the secondary air securely to theexhaust system by stopping the fuel supply to the one cylinder group orperforming the lean-A/F operation.

[0032] Further, the exhaust emission control device of theinternal-combustion engine of the present invention is an exhaustemission control device of an internal-combustion engine, which has aninjection valve for injecting a fuel directly into a combustion chamber,fuel injection control means for controlling fuel injection by means ofthe injection valve, and air-fuel ratio control means for controllingthe air-fuel ratio, comprising exhaust flow control means forrestraining an exhaust flow so as to enhance the effect of reduction ofthe delivery of harmful substances at the start of operation of theinternal-combustion engine, and secondary air supply means for supplyingsecondary air to the exhaust system of the internal-combustion engine atthe start of operation of the internal-combustion engine, the fuelinjection control means and the air-fuel ratio control means beingadapted to inject the fuel in a compression stroke of theinternal-combustion engine and to adjust the combustion air-fuel ratioto the theoretical air-fuel ratio or a rich air-fuel ratio,respectively, when the secondary air is supplied to the exhaust systemby means of the secondary air supply means.

[0033] A conventional cylinder-injection internal-combustion engine inwhich a fuel is injected directly into a combustion chamber is believedto be liable to plug smoldering, since the fuel is injected into aregion near a spark plug in compression-stroke injection. The applicanthereof conducted an experiment and found that compression-strokeinjection with the internal-combustion engine in a cold state, inparticular, was able to ensure a satisfactory spray condition, lessenthe plug smoldering, and generate plenty of CO. In feeding the secondaryair into the exhaust passages, based on this fact, the fuel is injectedin the compression stroke of the internal-combustion engine to generateCO so that the combustion air-fuel ratio is the theoretical air-fuelratio or a rich air-fuel ratio.

[0034] Thus, in the case where the secondary air is supplied and theexhaust flow is restrained (to raise the exhaust pressure) at the startof operation of the internal-combustion engine, the exhaust gas can bemade to contain plenty of CO. Since CO reacts more easily than HC does,the reaction in the exhaust system can be satisfactorily accelerated toimprove the exhaust emission control efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a schematic view of an exhaust emission control deviceof an internal-combustion engine according to first and fifthembodiments of the present invention;

[0036]FIG. 2 is a diagram showing a butterfly valve for use as anexhaust flow control device;

[0037]FIG. 3 is a flowchart showing a control routine of start controlaccording to the first embodiment;

[0038]FIG. 4 is a diagram showing time-based changes of the delivery(e.g., at a catalyst outlet) of HC for cases where the exhaust pressureis adjusted to a given pressure (e.g., 700 mmHg=933 hPa) and thecombustion A/F is adjusted to 10 (full line) and 12 (broken line),compared with a case where the combustion A/F is adjusted to 14 (dashedline);

[0039]FIG. 5 is a diagram showing relations between the combustionair-fuel ratio before secondary air supply and the HC delivery for eachexhaust pressure as the results of measurement in 10 seconds after thestart of operation of the internal-combustion engine;

[0040]FIG. 6 is a diagram showing the relation between the exhaustair-fuel ratio after secondary air supply to an exhaust system and theHC delivery under the exhaust pressure of 700 mmHg (933 hPa) as theresult of an experiment in 50 seconds after the start of operation ofthe internal-combustion engine;

[0041]FIG. 7 is a schematic view of an exhaust emission control deviceof an internal-combustion engine according to a second embodiment of thepresent invention;

[0042]FIG. 8 is a view showing an exhaust manifold;

[0043]FIG. 9 is a flowchart showing a control routine of start controlaccording to the second embodiment;

[0044]FIG. 10 is a schematic view of an exhaust emission control deviceof an internal-combustion engine according to third and fourthembodiments of the present invention;

[0045]FIG. 11 is a detailed view showing the configuration of an exhaustsystem of the engine according to the third embodiment;

[0046]FIG. 12 is a diagram showing a tandem-type butterfly valve for useas an exhaust flow control device;

[0047]FIG. 13 is a flowchart showing a control routine of start controlaccording to the third embodiment;

[0048]FIG. 14 is a detailed view showing the configuration of an exhaustsystem of an engine according to a modification 1 of the thirdembodiment;

[0049]FIG. 15 is a detailed view showing the configuration of an exhaustsystem of an engine according to a modification 2 of the thirdembodiment;

[0050]FIG. 16 is a detailed view showing the configuration of an exhaustsystem of the engine according to the fourth embodiment;

[0051]FIG. 17 is a flowchart showing a control routine of start controlaccording to the fourth embodiment;

[0052]FIG. 18 is a flowchart showing a control routine of start controlaccording to the fifth embodiment; and

[0053]FIG. 19 is a diagram showing time-based changes of the HCconcentration and exhaust gas temperature on the lower-stream side of acatalyst for a case (full line) where secondary air and a fuel aresupplied and injected in a compression stroke, respectively, comparedwith a case (broken line) where the fuel is injected in an intakestroke.

BEST MODE FOR CARRYING OUT THE INVENTION

[0054] A first embodiment will be described first.

[0055] Referring now to FIG. 1, there is shown an outline of an exhaustemission control device of an internal-combustion engine according tothe first embodiment of the present invention. The following is adescription of the configuration of this exhaust emission controldevice.

[0056] For example, a cylinder-injection spark-ignition gasoline engineis used as an engine body (hereinafter referred to simply as engine) 1that serves as an internal-combustion engine. In this engine, fuelinjection in a compression stroke (compression-stroke injection) can becarried out together with fuel injection in an intake stroke(intake-stroke injection) by changing the fuel injection mode. Thiscylinder-injection engine 1 can easily realize operation with a leanair-fuel ratio (lean-A/F operation), besides operation with atheoretical air-fuel ratio (stoichiometric) and operation with a richair-fuel ratio (rich-A/F operation).

[0057] As shown in the same drawing, a cylinder head 2 of the engine 1is fitted with an electromagnetic fuel injection valve 6 along with aspark plug 4 for each cylinder, whereby a fuel can be injected directlyinto a combustion chamber.

[0058] The spark plug 4 is connected with a spark coil 8 that outputshigh voltage. Further, the fuel injection valve 6 is connected with afuel supplier (not shown) having a fuel tank by means of a fuel pipe 7.More specifically, the fuel supplier is provided with a low-pressurefuel pump and a high-pressure fuel pump, whereby the fuel in the fueltank can be supplied at a low fuel pressure or a high fuel pressure tothe fuel injection valve 6, so that the fuel can be injected from thefuel injection valve 6 into the combustion chamber at a desired fuelpressure.

[0059] The cylinder head 2 is formed with intake ports for theindividual cylinders arranged substantially in the vertical directionand is connected with one end of an intake manifold 10 so as tocommunicate with each intake port. The intake manifold 10 is providedwith an electromagnetic throttle valve 14 that regulates the rate ofintake.

[0060] Further, the cylinder head 2 is formed with exhaust ports for theindividual cylinders arranged substantially in the horizontal directionand is connected with one end of an exhaust manifold 12 so as tocommunicate with each exhaust port.

[0061] Further, each exhaust port is connected with a secondary air pump(secondary air supply means) 16 by means of an air passage 17. Secondaryair can be supplied to each exhaust port as the secondary air pump 16 isactuated.

[0062] Since the cylinder-injection engine 1 is of a known type, adetailed description of its configuration is omitted.

[0063] An exhaust pipe (exhaust passage) 20 is connected to the otherend of the exhaust manifold 12.

[0064] The exhaust pipe 20 is fitted with a three-way catalyst 30 foruse as an exhaust emission control device. The three-way catalyst 30 hasan active noble metal, such as copper (Cu), cobalt (Co), silver (Ag),platinum (Pt), rhodium (Rh), or palladium (Pd), as its carrier.

[0065] As shown in the same drawing, moreover, the exhaust pipe 20 isprovided with an exhaust pressure sensor 22 for detecting the exhaustpressure and an O₂ sensor or A/F sensor 24.

[0066] Further, the exhaust pipe 20 is fitted with an exhaust flowcontrol device (exhaust flow control means) 40 in that part thereofwhich is situated on the lower-stream side of the three-way catalyst 30.

[0067] The exhaust flow control device 40 is a device that is designedto accelerate reduction of harmful substances (including NOx, smoke, H₂,etc. as well as unburned substances such as HC, CO, etc.) in exhaustgas, and is configured to be able to change the exhaust pressure,exhaust gas density, and/or exhaust flow rate (factor that causesaugmentation of the effect of reduction). More specifically, the exhaustflow control device 40 is composed of a closed on-off valve 42 that canadjust the flow area of the exhaust pipe 20.

[0068] The closed on-off valve 42 may be any of various types. In thiscase, a butterfly valve is used that can be adjust the flow area of theexhaust pipe 20 by rotating a valve disc 44 around a shaft 43 thatpenetrates the exhaust pipe 20, as shown in FIG. 2 that illustrates avalve-open state and a valve-closed state. The butterfly valve isprovided with an actuator 45, and the butterfly valve is opened orclosed as the valve disc 44 is rotated around the shaft 43 by means ofthe actuator 45.

[0069] An ECU 60 is provided with memories (ROM, RAM, nonvolatile RAM,etc.), central processing unit (CPU), timer counter, etc. The ECU 60carries out comprehensive control of the exhaust emission control deviceincluding the engine 1.

[0070] The input side of the ECU 60 is connected with the aforesaidvarious sensors, including the exhaust pressure sensor 22, O₂ sensor orA/F sensor 24, etc., and is supplied with detection information fromthese sensors.

[0071] On the other hand, the output side of the ECU 60 is connectedwith the aforesaid various output devices, including the fuel injectionvalve 6, spark coil 8, throttle valve 14, secondary air pump (secondaryair supply means) 16, actuator 45, etc. An injection quantity, fuelinjection timing, ignition timing, exhaust flow controlled variable,etc. that are computed in accordance with the detection information fromthe various sensors are delivered to the various output devices,individually. Thereupon, an appropriate quantity of fuel is injectedfrom the fuel injection valve 6 at a proper time, spark ignition iscarried out at a proper time by means of the spark plug 4, the secondaryair is supplied at a proper time, and the on-off valve 42 is controlledso that a desired exhaust flow controlled variable (e.g., target exhaustpressure) is obtained.

[0072] The following is a description of the operation of the exhaustemission control device according to the first embodiment of the presentinvention constructed in this manner, that is, cold start control of theengine 1 according to the first embodiment.

[0073] Referring to FIG. 3, there is shown a flowchart for a startcontrol routine according to the first embodiment. The start controlwill now be described with reference to the same drawing.

[0074] In Step S10, whether or not the engine 1 is in its cold state isdetermined. In this case, whether or not the cooling water temperatureis lower than a given temperature (e.g., 60° C.) is determined. If thedecision is No, it can be concluded that the engine 1 is in itswarming-up state, whereupon this routine is finished. If the decision isYes, on the other hand, it can be concluded that the engine 1 is in thecold state, whereupon the program advances to Step S12. The decision inStep S10 is not limited to the decision on whether or not the coolingwater temperature is lower than the given temperature, and mayalternatively be a decision on whether or not the elapsed time after thestart of operation is shorter than a given time (e.g., 50 sec), forexample.

[0075] In Step S12, whether or not an exhaust system is in its coldstate is determined. This decision depends on whether or not any of thefollowing conditions is met, for example:

[0076] elapsed time after start<given time (e.g., 4 sec),

[0077] one or more fulfillments of (engine speed>given rotational speed(e.g., 1,200 rpm)),

[0078] exhaust gas temperature<given temperature (e.g., 600° C.),

[0079] oil temperature<given temperature (e.g., 35° C.).

[0080] cooling water temperature<given temperature (e.g., 40° C.).

[0081] Since the exhaust system (exhaust port, exhaust manifold 12,etc.) can be easily brought to its warm state before the engine 1 isbrought to its warming-up state, the cold period of the exhaust systemis shorter than the cold period of the engine 1. Thus, the cold state ofthe exhaust system described herein can be discriminated from the coldstate of the engine 1. Therefore, threshold decision values for theaforesaid conditions are lower than threshold decision values for thewarming-up state of the engine 1.

[0082] If any of the aforesaid conditions is met so that it is concludedthat the decision in Step S12 is Yes, that is, the exhaust system is inthe cold state, the program then advances to Step S14.

[0083] In Step S14, whether or not an engine stop period was longer thana given time (e.g., 15 min) is determined. More specifically, whether ornot a time long enough to make the engine 1 cold has elapsed since thestoppage of the engine 1 is determined.

[0084] The given values as the threshold decision values for theaforesaid conditions used in Steps S12 and S14 may be fixed values.Alternatively, however, they may be map values that are optimizedaccording to operating conditions (elapsed time after start, enginerotational speed, engine stop period, volumetric efficiency, brake meaneffective pressure, exhaust gas temperature, oil temperature, coolingwater temperature, rate of intake, exhaust volume flow rate, exhaustmass flow rate, or one or more indexes associated with these values).

[0085] A phenomenon is confirmed such that reaction of harmfulsubstances, such as unburned substances, NOx, etc., cannot beaccelerated very much and the exhaust emission control performancelowers in the aforesaid manner so that the delivery of the harmfulsubstances (HC in the main) increases temporarily even if exhaust flowcontrol is carried out to raise the exhaust pressure and exhaust gasdensity in a period during which the exhaust system is in the cold state(see FIG. 4).

[0086] This phenomenon is supposed to occur for the following reason.Since the exhaust system temperature near the exhaust port is normallylow immediately after the start of operation of the internal-combustionengine (immediately after the start of cranking), exhaust gas is cooledto lower the exhaust gas temperature, so that reaction cannot beaccelerated very much even if the exhaust pressure is raised.

[0087] If the decisions in Steps S12 and S14 are Yes, according to thisexhaust emission control device, therefore, the combustion air-fuelratio (combustion A/F) is enriched in Step S16 lest the exhaust emissioncontrol performance be lowered in the period during which the exhaustsystem is in the cold state immediately after the start of operation ofthe engine 1. In this case, the combustion A/F is minimized to, forexample, 10 (A/F=10) with reference to FIG. 5. However, it is necessaryonly that the combustion A/F be a rich air-fuel ratio, and preferably,the combustion A/F should be restricted to the range from a combustionlimit air-fuel ratio to 13 (air-fuel ratio control means). In this case,the fuel injection is intake-stroke injection.

[0088] If the air-fuel ratio is enriched in this manner, the injectionquantity increases, so that the combustion heat release in thecombustion chamber increases, the combustion temperature rises, and thequantity of unburned substances increases. Accordingly, the probabilityof reaction of the unburned substances in the combustion chamber and theexhaust system, including the exhaust port, exhaust manifold, etc., isenhanced, so that the reaction is accelerated like a chain reaction.Thus, the exhaust system including the exhaust port, exhaust manifold,etc. in the cold state can be heated up in a short time, so that theexhaust gas temperature immediately after the start of operation can beprevented from lowering.

[0089] In the next step or Step S18, restraint of the exhaust flow isexecuted (exhaust flow control means). More specifically, the on-offvalve 42 is opened to restrain the exhaust flow, thereby raising theexhaust pressure. In this case, the actuator 45 is operated inaccordance with information from the exhaust pressure sensor 22, wherebythe exhaust pressure is raised to and kept at a given pressure (e.g.,700 mmHg=933 hPa).

[0090] Even immediately after the start of operation of the engine 1,therefore, the exhaust gas temperature can be prevented from lowering asthe exhaust pressure is kept high enough to lengthen the residence timeor reaction time of oxygen and the unburned substances. Thus, oxidationof HC, CO, etc. and reduction of NOx in the exhaust system can befavorably accelerated at a point of time immediately after the start ofoperation, and the delivery of HC, CO, NOx, etc. can be satisfactorilyrestrained from temporarily increasing immediately after the start ofoperation.

[0091] In Step S20, the secondary air is supplied from the secondary airpump 16 so that the exhaust air-fuel ratio (exhaust A/F) is a leanair-fuel ratio ranging from 18 to 22, in particular, in accordance withinformation from the O₂ sensor or A/F sensor 24 (secondary air supplymeans). Preferably, in this case, the exhaust A/F should be adjusted to20, as shown in FIG. 6.

[0092] Thus, if the secondary air supply is carried out so that theexhaust A/F ranges from 18 to 22 without failing to keep the exhaustpressure high enough, the oxidation and reduction in the exhaust systemare rapidly accelerated like a chain reaction, so that harmfulsubstances, such as HC, CO, NOx, etc., can be removed satisfactorily.

[0093] If any of the aforesaid conditions fails to be met because theelapsed time after the start of operation exceeds the given time (e.g.,4 sec), for example, so that the decision in Step S12 or S14 isconcluded to be No, on the other hand, the program then advances to StepS22. Thereupon, the combustion A/F is adjusted to, for example, 12(A/F=12) (air-fuel ratio control means).

[0094] Thus, if the decision in Step S12 or S14 is No, the combustionchamber and the exhaust system, including the exhaust port, exhaustmanifold 12, etc., can be concluded to be already off the cold state andin the warm state. In this state, therefore, the exhaust system need notbe heated up any more, and the combustion A/F is adjusted to a value alittle closer to the theoretical air-fuel ratio than to the combustionA/F (A/F=10) for the cold state.

[0095] Continuing the enrichment requires increase in the exhaust flowrate to compensate for reduction in torque. By making the aforesaidadjustment, however, the increase of the delivery of the harmfulsubstances that is involved in the increase of the exhaust flow rate canbe restrained, and the fuel-efficiency can be prevented from lowering.

[0096] Also in this case, the combustion A/F is adjusted to 12 (A/F=12),for example, and is a rich air-fuel ratio preferably ranging from thecombustion limit air-fuel ratio to 13 (air-fuel ratio control means).

[0097] After Step S22 is executed, the exhaust flow control is carriedout in the same manner as aforesaid and the secondary air is supplied sothat the exhaust air-fuel ratio (exhaust A/F) ranges from 18 to 22, inSteps S18 and S20.

[0098] Thus, the oxidation and reduction in the exhaust system continueto be rapidly accelerated like a chain reaction, and harmful substances,such as HC, CO, NOx, etc., continue to removed satisfactorily.

[0099] Thus, the delivery of HC, CO, NOx, and other harmful substancescan be reduced to improve the exhaust emission control efficiency byrestraining the exhaust flow, adjusting the combustion A/F to a richair-fuel ratio, and supplying the secondary air so that the exhaust A/Fis a lean air-fuel ratio, at the start of operation of the engine 1. Theexhaust emission control performance can be optimized by adjusting thecombustion A/F within the range from the combustion limit air-fuel ratioto 13 and supplying the secondary air so that the exhaust A/F rangesfrom 18 to 22, in particular. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0100] Referring to FIG. 4, there are shown time-based changes of thedelivery (e.g., at a catalyst outlet) of HC for cases where thesecondary air is supplied, the exhaust pressure is adjusted to a givenpressure (e.g., 700 mmHg=933 hPa), and the combustion A/F is adjusted to10 (full line) and 12 (broken line), compared with a case where thecombustion A/F is adjusted to 14 (dashed line). Immediately after thestart of operation, as seen from this drawing, the HC delivery isconsiderably reduced when the combustion A/F is 10, that is, if thecombustion A/F is closer to the combustion limit air-fuel ratio.Thereafter, the HC delivery has its minimum when the combustion A/F is12 or thereabout. At the start of operation of the engine 1, therefore,the delivery of harmful substances can be reduced without failing tooptimizing the exhaust emission control performance by adjusting thecombustion A/F to a rich air-fuel ratio, especially to a value withinthe range from the combustion limit air-fuel ratio to 13, such as 10 or12. By supplying the secondary air so that the exhaust A/F ranges from18 to 22, the delivery of the harmful substances can be further reducedwithout failing to optimizing the exhaust emission control performanceadditionally.

[0101] According to this arrangement, the combustion A/F is adjusted to10 immediately after the start of operation of the engine 1 and thenchanged into 12. However, the combustion A/F is not limited to thistwo-stage switching, and may be switched in three or more stages withinthe range of rich air-fuel ratios, depending on the elapsed time sincethe start of operation of the engine 1. Thus, the exhaust emissioncontrol performance can be optimized additionally. Preferably, in thiscase, the value of the combustion A/F should be gradually increased withtime. More specifically, the combustion A/F should be gradually changedfrom a rich air-fuel ratio into a value near the theoretical air-fuelratio. As this is done, the combustion A/F may be changed continuously.

[0102] The A/F values described in this specification (including theclaims) are values for domestic premium gasoline (theoretical air-fuelratio: 14.41). The individual A/F values imply the following equivalentratios ø :

A/F=10→ø≈1.441,

A/F=12→ø≈1.201,

A/F=13→ø≈1.108,

A/F=14→ø≈1.029,

A/F=18→ø≈0.801,

A/F=20 →ø26 0.721,

A/F=22 →ø≈0.655.

[0103] The following is a description of a second embodiment.

[0104] The second embodiment differs from the first embodiment in thatno secondary air pump is used and that the engine 1 is a multi-cylinderengine. A description of portions that are shared with the firstembodiment is omitted herein, and only different portions will bedescribed below.

[0105] Referring to FIG. 7, there is shown an outline of an exhaustemission control device of an internal-combustion engine according tothe second embodiment of the present invention. In this secondembodiment, a cylinder-injection spark-ignition four-cylinder gasolineengine is used as the engine 1, for example.

[0106] In this case, moreover, a dual-type exhaust manifold system suchas the one shown in FIG. 8 is used as the exhaust manifold 12.Alternatively, the exhaust manifold 12 may be a single-type exhaustmanifold system or a clamshell-type exhaust manifold system.

[0107] The following is a description of the operation of the exhaustemission control device of the internal-combustion engine according tothe second embodiment of the present invention constructed in thismanner, that is, cold start control of the engine 1 according to thesecond embodiment.

[0108] Referring to FIG. 9, there is shown a flowchart for a startcontrol routine according to the second embodiment. The start controlwill now be described with reference to the same drawing. The followingis a description of only those portions which are different from theflowchart of FIG. 3.

[0109] When the exhaust flow is restrained in Steps S10 to S16 or S22,according to this second embodiment, fuel supply to any one of cylinders#1 to #4 is stopped (fuel-cut) in Step S20′ (secondary air supplymeans), and the combustion air-fuel ratio for the other three cylindersis kept at the aforesaid rich air-fuel ratio. Since the engine 1 is afour-cylinder engine, the fuel supply for one cylinder is cut with everyfour cycles. For example, the fuel supply for the cylinder #1 is cut,and the fuel is injected through the fuel injection valve 6 into thecylinders #2 to #4 so that a rich air-fuel ratio is obtained. In thiscase, the fuel injection into the cylinders #2 to #4 is intake-strokeinjection.

[0110] Thus, in this case, the fuel supply is controlled so that thefuel in a rich air-fuel ratio is supplied to the next cylinder after thefuel-cut for one cylinder is carried out.

[0111] Although the fuel-cut is carried out for only one of thecylinders in the case described above, the fuel-cut may be carried outfor any two of the cylinders with every two cycles.

[0112] If the fuel-cut is thus carried out for any of the cylinderswhile the fuel is supplied to other cylinders so that the air-fuel ratiois a rich air-fuel ratio, only air (exhaust air) is discharged into theexhaust manifold 12 from some of the cylinders that are subjected to thefuel-cut, while great quantities of unburned substances (HC, CO, etc.)that are attributable to incomplete combustion are discharged into theexhaust manifold 12 from the other cylinders to which the fuel isexcessively supplied to obtain the rich air-fuel ratio.

[0113] If oxygen and the unburned substances are supplied to the exhaustmanifold 12, oxidation is accelerated satisfactorily in the exhaustsystem, including the exhaust manifold 12 and the exhaust pipe 20, inthe presence of sufficient oxygen with the exhaust flow restrained.

[0114] Thus, as in the case of the first embodiment, the delivery of HC,CO, NOx, and other harmful substances can be reduced to improve theexhaust emission control efficiency. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0115] Although the exhaust air is supplied through the execution offuel-cut in the case described above, the fuel-cut may be replaced withlean-A/F operation such that exhaust gas containing plenty of surplusoxygen is supplied as the exhaust air (exhaust oxygen quantityincreasing means). The same effect as aforesaid can be also obtained inthis case.

[0116] The following is a description of a third embodiment.

[0117] A description of those portions of the third embodiment which areshared with the first embodiment is also omitted, and only differentportions will be described below.

[0118] Referring to FIG. 10, there is shown an outline of an exhaustemission control device of an internal-combustion engine according tothe third embodiment of the present invention. In this secondembodiment, as in the second embodiment described above, acylinder-injection spark-ignition four-cylinder gasoline engine is alsoused as the engine 1, for example.

[0119] In this case, a dual-type exhaust manifold system is used as theexhaust manifold 12.

[0120] Referring to FIG. 11, there is shown a detailed view of theexhaust system of the engine 1. The configuration of the exhaust systemof the engine 1 according to the third embodiment will now be describedwith reference to the same drawing.

[0121] Combustion in the engine 1 is carried out for #1, #3, #4 and #2in the order named. In order to avoid exhaust interference, in theexhaust manifold 12 that is formed of the dual-type exhaust manifoldsystem, therefore, the respective exhaust passages of the cylinders #1and #4 (one cylinder group) that are not continuous with each other jointogether to form one exhaust passage, while the respective exhaustpassages of the cylinders #2 and #3 (other cylinder group) join togetherto form another exhaust passage. Thus, the exhaust manifold 12, which isformed of the dual-type exhaust manifold system, has two exhaustpassages at its outlet.

[0122] As shown in the same drawing, the exhaust pipe 20 is divided intwo, an exhaust pipe 20 a and an exhaust pipe 20 b, by means of a screenor the like. The exhaust pipe 20 a is connected to the exhaust passagesfrom the cylinders #1 and #4, while the exhaust pipe 20 b is connectedto the exhaust passages from the cylinders #2 and #3.

[0123] The exhaust pipe 20 is fitted with an exhaust flow control device140 that controls the flow of exhaust gas in the exhaust pipe 20 thatincludes the exhaust pipe 20 a and the exhaust pipe 20 b.

[0124] The exhaust flow control device 140, like the aforesaid exhaustflow control device 40, is configured to be able to carry out therestraint of the exhaust flow, that is, the rise of the exhaustpressure, increase of the exhaust gas density, and/or reduction of theexhaust flow rate. More specifically, the exhaust flow control device140 is composed of a closed on-off valve (exhaust flow control means)142 that can adjust the respective flow areas of the exhaust pipe 20 aand the exhaust pipe 20 b.

[0125] The closed on-off valve 142 may be any of various types. In thiscase, a tandem-type butterfly valve such as the one schematically shownin FIG. 12 is used for the purpose.

[0126] This tandem-type butterfly valve is constructed so that a valvedisc 144 a corresponding to the exhaust pipe 20 a and a valve disc 144 bcorresponding to the exhaust pipe 20 b are independent of each other,and both are fixed to a rotating shaft 143 so as to be rotatable insynchronism with the rotating shaft 143. Thus, the closed on-off valve142, as the tandem-type butterfly valve, has the two valve discs 144 aand 144 b that are integrally formed sharing the one rotating shaft 143.

[0127] An actuator 145 is connected to the rotating shaft 143. Thebutterfly valve is opened and closed as the rotating shaft 143 isrotated by means of the actuator 145.

[0128] More specifically, the rotating shaft 143 is composed of a shaftbody 143 a fitted with the valve disc 144 a and a shaft body 143 bfitted with the valve disc 144 b. The shaft bodies 143 a and 143 b areconnected in series by means of a spring 143 c so that the valve discs144 a and 144 b are shifted for a given angle around the rotating shaft143. Thus, the butterfly valve is constructed so that the reduction ofthe flow area of the exhaust pipe 20 a constricted by means of the valvedisc 144 a is greater than the reduction of the flow area of the exhaustpipe 20 b constricted by means of the valve disc 144 b when the rotatingshaft 143 is rotated to the valve-closing side, and that both the valvediscs 144 a and 144 b are fully opened against the spring force when therotating shaft 143 is rotated to its full-open position on thevalve-opening side.

[0129] The exhaust pipe 20 is integrated on the lower-stream side ofclosed on-off valve 142, and the integrated portion of the exhaust pipe20 is fitted with the three-way catalyst 30.

[0130] The respective exhaust passages of the cylinders #1 and #2 areconnected to the respective exhaust passages of the cylinders #3 and #4by means of small-diameter communicating channels 18 and 19,respectively. The communicating channels 18 and 19 may be formed in thecylinder head 2 in a manner such that the respective exhaust ports ofthe cylinders #1 and #4 communicate with the respective exhaust ports ofthe cylinders #2 and #3, respectively.

[0131] The following is a description of the operation of the exhaustemission control device according to the third embodiment of the presentinvention constructed in this manner, that is, cold start control of theengine 1 according to the third embodiment.

[0132] Referring to FIG. 13, there is shown a flowchart for a startcontrol routine according to the third embodiment. The start controlwill now be described with reference to the same drawing. The followingis also a description of only those portions which are different fromthe flowchart of FIG. 3.

[0133] After the processes of Steps S10 to S16 or S22 are executed,according to this third embodiment, the actuator 45 of the exhaust flowcontrol device 140 is activated to close the closed on-off valve 142 inStep S18. Thus, both the valve discs 144 a and 144 b of the tandem-typebutterfly valve are closed to reduce the respective flow areas of boththe exhaust pipes 20 a and 20 b.

[0134] In Step S120, fuel supply to the cylinders #1 and #4 (onecylinder group) is stopped, that is, fuel-cut (exhaust oxygen quantityincreasing means) is executed to discharge only air (exhaust air) intothe exhaust pipe 20 a. On the other hand, rich-A/F operation is carriedout in the cylinders #2 and #3 (other cylinder group) so that unburnedsubstances are discharged into the exhaust pipe 20 b with the combustionair-fuel ratio kept at the rich air-fuel ratio. In this case, the fuelinjection into the cylinders #2 and #3 is intake-stroke injection.

[0135] Thus, the exhaust flow in the exhaust pipes 20 a and 20 b isrestrained. Since the closed on-off valve 142 is constructed so that thereduction of the flow area of the exhaust pipe 20 a constricted by meansof the valve disc 144 a is greater than the reduction of the flow areaof the exhaust pipe 20 b constricted by means of the valve disc 144 b,as mentioned before, however, the degree of restraint of the exhaustflow in the exhaust pipe 20 a is higher than the degree of restraint ofthe exhaust flow in the exhaust pipe 20 b, so that the exhaust pressurein the exhaust pipe 20 a is higher than the exhaust pressure in theexhaust pipe 20 b.

[0136] Based on the resulting pressure difference, exhaust air in theexhaust passage of the cylinder #1 flows into the exhaust passage of thecylinder #3 through the communicating channel 18, exhaust air in theexhaust passage of the cylinder #4 flows into the exhaust passage of thecylinder #2 through the communicating channel 19, and exhaust air on theside of the exhaust pipe 20 a discharged from the cylinders #1 and #4 isintroduced into the respective exhaust passages of the cylinders #2 and#3, that is, into the exhaust pipe 20 b.

[0137] Thus, in the case of the third embodiment, air can be easilysupplied to the exhaust passages of the combustion cylinder group(cylinders #2 and #3 in this case) by carrying out fuel-cut for thecylinders #1 and #4 (one cylinder group) and raising the exhaustpressure in the exhaust pipe 20 a.

[0138] In the case where the exhaust pressure in the exhaust pipe 20 bis also raised to accelerate reaction in the exhaust system byrestraining the exhaust flow, in particular, the secondary air cannot besatisfactorily mixed into the exhaust gas if the discharge pressure ofthe secondary air pump is lower than the exhaust pressure. However,there is a pressure difference is created such that the exhaust pressurein the exhaust passages from the cylinders #1 and #4 (one cylindergroup), that is, in the exhaust pipe 20 a, is higher than the exhaustpressure in the exhaust passages from the cylinders #2 and #3 (othercylinder group), that is, in the exhaust pipe 20 b. If the exhaustpressures in the respective exhaust passages of the combustion cylindergroups are raised by restraining the exhaust flow, therefore, air can besecurely supplied to the exhaust passages of the combustion cylindergroups without using any expensive high-output secondary air pump.

[0139] If air is thus supplied to the exhaust passages of the combustioncylinder groups from which unburned substances are discharged, oxidationis accelerated satisfactorily in the exhaust system, including theexhaust manifold 12 and the exhaust pipe 20, in the presence ofsufficient oxygen with the exhaust flow restrained.

[0140] Thus, as in the case of the first embodiment, the delivery of HC,CO, NOx, and other harmful substances can be reduced to improve theexhaust emission control efficiency. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0141] Referring to FIGS. 14 and 15, there are shown modifications ofthe third embodiment. The following is a description of modifications 1and 2 of the third embodiment.

[0142] In the modification 1, as shown in FIG. 14, the respectiveexhaust passages of the cylinders #1 and #2 are connected to therespective exhaust passages of the cylinders #3 and #4 by means of thesmall-diameter communicating channels 18 and 19, respectively, as in thecase of FIG. 11.

[0143] In the modification 1, moreover, a closed on-off valve (throttlemeans) 142 a′ corresponding to the exhaust pipe 20 a is independentlyset on the upper-stream side of the three-way catalyst 30, while aclosed on-off valve (exhaust flow control means) 142 b′ corresponding tothe exhaust pipe 20 b is independently set in that part of the exhaustpipe 20, not of the exhaust pipe 20 b, which is situated on thelower-stream side of the three-way catalyst 30. Thus, in this case, theclosed on-off valves 142 a′ and 142 b′ constitute an exhaust flowcontrol device 140′. For example, butterfly valves are also used for theclosed on-off valves 142 a′ and 142 b′.

[0144] Thus, in the modification 1, the flow of the exhaust gas from thecylinders #2 and #3, the combustion cylinder group, is restrained withina range including the three-way catalyst 30.

[0145] These closed on-off valves 142 a′ and 142 b′ are constructed sothat the reduction of the flow area of the exhaust pipe 20 a constrictedby means of the closed on-off valve 142 a′ is greater than the reductionof the flow area of that part of the exhaust pipe 20 which is situatedon the lower-stream side of the three-way catalyst 30, constricted bymeans of the closed on-off valve 142 b′.

[0146] When both the closed on-off valves 142 a′ and 142 b′ are closed,also in the modification 1, therefore, the degree of restraint of theexhaust flow in the exhaust pipe 20 a is higher than the degree ofrestraint of the exhaust flow in the exhaust pipe 20 b, as in the caseof FIG. 11. Based on the resulting pressure difference, exhaust airdischarged from the cylinders #1 and #4 (one cylinder group) is securelysupplied to the respective exhaust passages of the cylinders #2 and #3(other cylinder group).

[0147] Thus, as in the case described above, oxidation is acceleratedsatisfactorily in the exhaust system, and the delivery of HC, CO, NOx,and other harmful substances can be reduced to improve the exhaustemission control efficiency. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0148] In the case of FIG. 11, the valve discs 144 a and 144 b of theclosed on-off valve 142 are coupled by means of the rotating shaft 143,so that exhaust heat in the exhaust pipe 20 b heated by exhaust heat-upis transmitted from the valve disc 144 b to the valve disc 144 a andcooled by means of. the exhaust air in the exhaust pipe 20 a. If theclosed on-off valves are arranged independently, as in the modification1, however, such a heat loss can be prevented, so that exhaust heat-upcan be carried out satisfactorily in the exhaust pipe 20 b, and thethree-way catalyst 30 can be activated earlier.

[0149] If the closed on-off valve 142 b′ is located on the lower-streamside of the three-way catalyst 30, moreover, the exhaust gastemperature, having been very high on the upper-stream side of thethree-way catalyst 30 owing to the exhaust heat-up in the exhaust pipe20 b, lowers on the lower-stream side of the three-way catalyst 30because the exhaust heat is used for the heat-up of the three-waycatalyst 30. Accordingly, the closed on-off valve 142 b′ can beprevented from being overheated, so that the durability of the closedon-off valve 142 b′ can be improved.

[0150] In the modification 2, as shown in FIG. 15, the respectiveexhaust passages of the cylinders #1 and #2 are connected to therespective exhaust passages of the cylinders #3 and #4 by means of thesmall-diameter communicating channels 18 and 19, respectively, as in thecase of FIG. 11 also.

[0151] In the modification 2, moreover, that part of the exhaust pipe 20which is situated on the lower-stream side of the three-way catalyst 30is bent and extends for a fixed range along the exhaust pipe 20 a. Thisfixed-range portion is fitted with an exhaust flow control device 140″that controls exhaust flows in the exhaust pipe 20 a and that part ofthe exhaust pipe 20 which is situated on the lower-stream side of thethree-way catalyst 30.

[0152] In this case, as in the case of FIG. 11, a closed on-off valve(exhaust flow control means) 142″ formed of a tandem-type butterflyvalve is used as the exhaust flow control device 140″.

[0153] When the closed on-off valve 142″ is closed, also in themodification 2, therefore, the degree of restraint of the exhaust flowin the exhaust pipe 20 a is higher than the degree of restraint of theexhaust flow in the exhaust pipe 20 b. Based on the resulting pressuredifference, exhaust air discharged from the cylinders #1 and #4. (onecylinder group) is securely supplied to the respective exhaust passagesof the cylinders #2 and #3, the combustion cylinder group (othercylinder group).

[0154] Thus, as in the case described above, oxidation is acceleratedsatisfactorily in the exhaust system, and the delivery of HC, CO, NOx,and other harmful substances can be reduced to improve the exhaustemission control efficiency. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0155] As in the case of the modification 1, furthermore, the exhaustheat in the exhaust pipe 20 b heated by exhaust heat-up is transmittedto the exhaust pipe 20 a and is never cooled. Thus, a heat loss can beprevented, so that exhaust heat-up can be carried out satisfactorily inthe exhaust pipe 20 b, and the three-way catalyst 30 can be activatedearlier.

[0156] As in the case of the modification 1, moreover, the exhaust gastemperature is lowered on the lower-stream side of the three-waycatalyst 30 because heat from the exhaust heat-up is used for theheat-up of the three-way catalyst 30. Accordingly, the closed on-offvalve 142″ can be prevented from being overheated, so that thedurability of the closed on-off valve 142″ can be improved.

[0157] In the modification 2, compared with the modification 1,moreover, the closed on-off valve 142″ is formed integrally withoutbeing split, the cost can be cut down without failing to secureadvantageous effects on exhaust heat-up, durability, etc.

[0158] In this case, fuel-cut is carried out for the cylinders #1 and#4, and combustive operation for the cylinders #2 and #3. In contrastwith this, however, fuel-cut may be carried out for the cylinders #2 and#3, and combustive operation for the cylinders #1 and #4. If the exhaustpipe from the cylinder group that is subjected to fuel-cut and theexhaust pipe from the cylinder group that undergoes combustive operationare arranged independently of each other, moreover, only one of thecylinders or any three of them may be subjected to fuel-cut. Further,the number of cylinders to be subjected to fuel-cut may be changeddepending on the required output or oscillation mode of the engine 1. Ifthe cylinders to be subjected to fuel-cut and the cylinders to besubjected to combustive operation are alternately changed with everygiven period, the state of the combustion chamber of each cylinder, theheat-up state of the three-way catalyst 30, etc. can be kept constant.

[0159] Although the exhaust air is supplied through the execution offuel-cut in the case described above, the fuel-cut may be replaced withlean-A/F operation such that exhaust gas containing plenty of surplusoxygen is supplied as the exhaust air to the combustion group side(exhaust oxygen quantity increasing means). The same effect as aforesaidcan be also obtained in this case.

[0160] Making the flow area reduction by means of the valve disc 144 aor the closed on-off valve 142 a′ in the exhaust pipe 20 a on the sidefor the increase of exhaust oxygen greater than the flow area reductionby means of the other valve disc 144 b or the closed on-off valve 142 b′involves fully cutting off the exhaust pipe 20 a by means of the valvedisc 144 a or the closed on-off valve 142 a′. Thus, the secondary aircan be securely supplied, and the three-way catalyst 30 on thelower-stream side can be prevented from being cooled by means oflow-temperature air.

[0161] The following is a description of a fourth embodiment.

[0162] A description of those portions of the fourth embodiment whichare shared with the first and third embodiments is also omitted, andonly different portions will be described below.

[0163]FIG. 10 is applied to the fourth embodiment, and acylinder-injection spark-ignition four-cylinder gasoline engine is usedas the engine 1, for example, as in the second and third embodimentsdescribed above.

[0164] Referring to FIG. 16, there is shown a detailed view of theexhaust system of the engine 1 according to the fourth embodiment of thepresent invention. The configuration of the exhaust system of the engine1 according to the fourth embodiment will now be described withreference to the same drawing.

[0165] In the fourth embodiment, the communicating channels 18 and 19are fitted with a small-sized air pump 16′ with a discharge pressure ofabout 150 mmHg (200 hPa), which constitutes an exhaust air system. Morespecifically, the air pump 16′ is configured and arranged so as toforce-feed exhaust gas from the exhaust passage of the cylinder #1 tothe exhaust passage of the cylinder #3 and from the exhaust passage ofthe cylinder #4 to the exhaust passage of the cylinder #2.

[0166] The closed on-off valve (exhaust flow control means) 42 for useas the exhaust flow control device 40 is set on the lower-stream side ofthe three-way catalyst 30, whereby the flow of the exhaust gas in theexhaust pipe 20 that includes the exhaust pipe 20 a and the exhaust pipe20 b.

[0167] The following is a description of the operation of the exhaustemission control device according to the fourth embodiment of thepresent invention constructed in this manner, that is, cold startcontrol of the engine 1 according to the fourth embodiment.

[0168] Referring to FIG. 17, there is shown a flowchart for a startcontrol routine according to the fourth embodiment. The start controlwill now be described with reference to the same drawing. The followingis also a description of only those portions which are different fromthe flowchart of FIG. 3.

[0169] After the processes of Steps S10 to S16 or S22 are executed,according to this fourth embodiment, the closed on-off valve 42 of theexhaust flow control device 40 is closed in Step S18, whereupon theexhaust flow is restrained.

[0170] In Step S120, the cylinders #1 and #4 (one cylinder group) issubjected to fuel-cut (exhaust oxygen quantity increasing means) todischarge only air (exhaust air) into the exhaust pipe 20 a. On theother hand, rich-A/F operation is carried out in the cylinders #2 and #3(other cylinder group) so that unburned substances are discharged intothe exhaust pipe 20 b with the combustion air-fuel ratio kept at therich air-fuel ratio. In this case, the fuel injection into the cylinders#2 and #3 is intake-stroke injection.

[0171] In Step S122, the air pump 16′ is actuated to mix the exhaust airdischarged from the cylinders #1 and #4 into the exhaust gas from thecylinders #2 and #3, the combustion cylinder group.

[0172] Thus, in the case of the fourth embodiment, the exhaust air canbe easily supplied to the exhaust passages of the combustion cylindergroup despite the absence of the pressure difference between therespective exhaust pressures of the exhaust pipes 20 a and 20 b, whichis present in the third embodiment. Even in the case where exhaustpressure is raised by restraining the exhaust flow, therefore, theexhaust air can be securely supplied to the exhaust passages of thecombustion cylinder group by keeping the air pressure on the intake sideof the air pump 16′ and the exhaust pressure on the discharge side, thatis, the exhaust air pressure in the exhaust pipe 20 a and the exhaustpressure in the exhaust pipe 20 b, on the same level. In this case, theair pump 16′ is small-sized, so that it can be easily obtained withoutentailing high cost.

[0173] Thus, as in the case of the first embodiment, the delivery of HC,CO, NOx, and other harmful substances can be reduced to improve theexhaust emission control efficiency. Since plenty of heat of reaction isgenerated, moreover, the exhaust gas temperature can be kept high, andthe three-way catalyst 30 can be activated early.

[0174] The air pump 16′ may be provided in the communicating channels 18and 19 of the third embodiment.

[0175] The following is a description of a fifth embodiment.

[0176] A description of those portions of the fifth embodiment which areshared with the first embodiment is also omitted, and only differentportions will be described below.

[0177]FIG. 1 is applied to the fifth embodiment, and acylinder-injection spark-ignition gasoline engine is used as the engine1, for example, as in the first embodiment described above.

[0178] The following is a description of the operation of the exhaustemission control device according to the fifth embodiment of the presentinvention constructed in this manner, that is, cold start control of theengine 1 according to the fifth embodiment.

[0179] Referring to FIG. 18, there is shown a flowchart for a startcontrol routine according to the fifth embodiment. The start controlwill now be described with reference to the same drawing. The followingis also a description of only those portions which are different fromthe flowchart of FIG. 3.

[0180] After the processes of Steps S10 to S14 are executed, accordingto this fifth embodiment, the combustion A/F is adjusted to, forexample, 10 (A/F=10) in Step S16, and the combustion A/F is adjusted to,for example, 12 (A/F=12) in Step S22. In this state, however, it isnecessary only that the combustion A/F be a rich air-fuel ratio ortheoretical air-fuel ratio, and preferably, the combustion A/F should berestricted to the range from a combustion limit air-fuel ratio to 13(air-fuel ratio control means).

[0181] In Step S18, the on-off valve 42 of the exhaust flow controldevice 40 is closed, whereupon the exhaust flow is restrained.

[0182] Then, in Step S19, the fuel injection mode is switched over to acompression-stroke injection mode so that the fuel can be injected inthe compression stroke. Preferably, in this case, the compression-strokeinjection end timing should be adjusted to BTDC 60° or thereabout.

[0183] In Step S20, moreover, the secondary air is supplied from thesecondary air pump 16 so that the exhaust A/F is a lean air-fuel ratioranging from 18 to 22, in particular (secondary air supply means).Preferably, the exhaust A/F should be adjusted to 20.

[0184] Thus, air is introduced into the exhaust passages, and thecombustion A/F is adjusted to a rich air-fuel ratio or theoreticalair-fuel ratio. Unburned substances exist together with oxygen in theair in the exhaust system, and oxygen and the unburned substances reactin the exhaust system. As this is done, the fuel is injected in thecompression stroke. Thereupon, the spray condition of the fuel becomesfavorable, as mentioned before, so that the spark plug 4 smolders lessespecially when the engine 1 is in the cold state. Therefore, theexhaust gas contains plenty of CO. Since CO reacts more easily than HCdoes, the reaction in the exhaust system is accelerated satisfactorily.

[0185] Referring to FIG. 19, there is shown a time chart representingtime-based changes of the HC concentration and exhaust gas temperatureon the lower-stream side of the catalyst for a case (full line) wherethe secondary air and the fuel are supplied and injected in thecompression stroke, respectively, compared with a case (broken line)where the fuel is injected in the intake stroke. If the secondary airand the fuel are thus supplied and injected in the compression stroke,respectively, the delivery of HC can be satisfactorily reduced toincrease the exhaust gas temperature immediately after the start ofoperation (when motoring is switched over to firing).

[0186] Thus, the exhaust emission control efficiency can be improved,and the three-way catalyst 30 can be activated early.

[0187] According to the fifth embodiment, the secondary air is suppliedby means of the secondary air pump 16. Alternatively, however, theexhaust air may be supplied in the manner described in connection withthe second to fourth embodiments.

[0188] Although the embodiments have been described herein, the presentinvention is not limited to the embodiments described above.

[0189] According to the embodiments described above, for example, theclosed on-off valves 42, 142, 142′ and 142″ are used as the exhaust flowcontrol devices 40, 140, 140′ and 140″, respectively. If theintake/exhaust system is provided with a turbocharger, however, a wastegate valve of the turbocharger may be used in place of the closed on-offvalve 42 or the like.

[0190] Further, the secondary air may be supplied by utilizing the boostpressure of the turbocharger.

[0191] According to the embodiments described above, moreover, thecylinder-injection gasoline engine is used as the engine 1.Alternatively, however, the engine 1 may be a diesel engine. For thefirst to fourth embodiments, moreover, it may be a manifold-injectiongasoline engine.

1. An exhaust emission control device of an internal-combustion engine,comprising: exhaust flow control means for restraining an exhaust flowso as to enhance the effect of reduction of the delivery of harmfulsubstances at the start of operation of the internal-combustion engine;secondary air supply means for supplying secondary air to an exhaustsystem of said internal-combustion engine at the start of operation ofsaid internal-combustion engine; and air-fuel ratio control means foradjusting the combustion air-fuel ratio of said internal-combustionengine to a rich air-fuel ratio at the start of operation of saidinternal-combustion engine.
 2. An exhaust emission control device of aninternal-combustion engine according to claim 1, wherein said air-fuelratio control means controls the combustion air-fuel ratio of saidinternal-combustion engine so that the combustion air-fuel ratio is notlower than a combustion limit air-fuel ratio and not higher than
 13. 3.An exhaust emission control device of an internal-combustion engineaccording to claim 1, wherein said secondary air supply means suppliesthe secondary air so that the exhaust air-fuel ratio obtained after thesecondary air supply is a lean air-fuel ratio.
 4. An exhaust emissioncontrol device of an internal-combustion engine according to claim 3,wherein said secondary air supply means supplies the secondary air sothat the exhaust air-fuel ratio obtained after the secondary air supplyranges from 18 to
 22. 5. An exhaust emission control device of aninternal-combustion engine according to claim 1, wherein saidinternal-combustion engine is a multi-cylinder internal-combustionengine, and said secondary air supply means supplies the secondary airby stopping fuel supply to some of cylinders or carrying out lean-A/Foperation.
 6. An exhaust emission control device of aninternal-combustion engine according to claim 1, wherein saidinternal-combustion engine is a multi-cylinder internal-combustionengine and comprises two exhaust passages provided independently foreach of two cylinder groups into which cylinders of said multi-cylinderinternal-combustion engine are divided and communicating channelsconnecting the two exhaust passages, and wherein said secondary airsupply means includes exhaust oxygen quantity increasing means forincreasing the quantity of oxygen in exhaust gas discharged from one ofsaid two cylinder groups, and supplies the secondary air as said exhaustcontrol means restrains the exhaust flow so that the degree of restraintof the flow of the exhaust gas discharged from said one cylinder groupis higher than the degree of restraint of the flow of the exhaust gasdischarged from the other cylinder group when the quantity of oxygen inthe exhaust gas discharged from said one cylinder group is increased bymeans of said exhaust oxygen quantity increasing means.
 7. An exhaustemission control device of an internal-combustion engine according toclaim 1, wherein said internal-combustion engine is a multi-cylinderinternal-combustion engine and comprises two exhaust passages providedindependently for each of two cylinder groups into which cylinders ofsaid multi-cylinder internal-combustion engine are divided andcommunicating channels connecting the two exhaust passages, and whereinsaid secondary air supply means includes exhaust oxygen quantityincreasing means for increasing the quantity of oxygen in exhaust gasdischarged from one of said two cylinder groups and an air pump attachedto said communicating channels and capable of force-feeding the exhaustgas from the exhaust passages of said one cylinder group to the exhaustpassages of the other cylinder group.
 8. An exhaust emission controldevice of an internal-combustion engine according to claim 6 or 7,wherein said exhaust oxygen quantity increasing means stops fuel supplyto one of said two cylinder groups or performs lean-A/F operation.
 9. Anexhaust emission control device of an internal-combustion engine, whichhas an injection valve for injecting a fuel directly into a combustionchamber, fuel injection control means for controlling fuel injection bymeans of said injection valve, and air-fuel ratio control means forcontrolling the air-fuel ratio, comprising: exhaust flow control meansfor restraining an exhaust flow so as to enhance the effect of reductionof the delivery of harmful substances at the start of operation of theinternal-combustion engine; and secondary air supply means for supplyingsecondary air to an exhaust system of said internal-combustion engine atthe start of operation of said internal-combustion engine, said fuelinjection control means and said air-fuel ratio control means beingadapted to inject the fuel in a compression stroke of theinternal-combustion engine and to adjust the combustion air-fuel ratioto the theoretical air-fuel ratio or a rich air-fuel ratio,respectively, when the secondary air is supplied to the exhaust systemby means of said secondary air supply means.